helper/command: simplify script_command_args_alloc()

The output parameter nwords is always equal to the input parameter
argc, when the function succeeds.

Drop the parameter nwords and let the caller use directly the
value in argc.

While there, convert some 'unsigned' to 'unsigned int'.

Change-Id: Ie3d8ce1351792f3c07fe39cdcbcd180fd24dc928
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8054
Tested-by: jenkins